This invention relates to an optical communications network which is defined by a number of interconnected nodes. The invention is particularly directed to a network in which protection paths are provided, which enable communication between pairs of nodes despite rupture of the main signal carrying fibre between those nodes.
There are various known architectures for providing signal protection. One possible way to provide a protection path between nodes is, for each adjacent pair of nodes of the network, to provide an additional protection cable between the pair of nodes which preferably follows a different path around the network to the main signal carrying fibre joining those nodes. This ensures that the protection cable is not susceptible to the same failure as the main cable. This type of dedicated protection scheme results in a large number of additional cables extending around the network, and these additional cables are normally unused. This approach is therefore bandwidth inefficient, although it is extremely simple to implement.
A known improvement to this approach is a ring configuration it which adjacent nodes are connected together by pairs of cablesxe2x80x94a working cable and a protection cable. This enables the protection channels to be shared. This configuration has been considered for coupling nodes together which are each arranged to add or drop signals in standard SONET format to or from the network. The signals are provided from the service platform on two channelsxe2x80x94a working channel and a protection channel. The nodes enable electrical switching of the entire signal carried by the working fibres onto the protection cables. This enables a span switch (in which for one section of the network between adjacent nodes, the protection fibre is employed instead of the main fibre) or a ring switch (in which signals for communication between adjacent nodes are redirected all around the network using the protection cables) to be implemented.
It is also known to couple nodes in a ring via two multiplexed communication paths providing for transmission in opposite directions around the ring. In normal operation communications are effected between the nodes in both directions via the two paths.
In the presence of a fault such as a fiber cut, this is detected in the two nodes immediately adjacent to the fault, and communications are maintained via both paths forming a folded loop, signals being coupled between the paths at these two nodes adjacent to the fault. Such systems are known as bidirectional line switched ring (BLSR) systems, and typically serve for communicating SONET signals in which case they are commonly referred to as SONET ring systems.
The switching operation in a BLSR ring requires low computational overhead. Many BLSR rings can be coupled together to define a network, and for a signal path across the network, each ring within the path provides the protection switching for that part of the signal path. When such rings are connected together, the nodes in adjacent rings are typically connected together using the tributary connections of the nodes. This means that the working and protection paths of each ring are maintained, and switching of signals between rings takes place at the nodes. U.S. Pat. No. 5,159,595 describes the BLSR architecture is detail and is incorporated herein as reference material.
In order to route a signal across such a network, a switching operation is carried out at the nodes, and protection is the automatic result of the ring structure. In other words, each span between nodes of the signal path is associated with a specific ring, which provides the protection capability.
BLSR systems have disadvantages in that they have a high utilization of optical fibers and do not provide for 1:N (N greater than 1) protection (i.e. protection of N working channels using one protection channel).
Mesh based network architectures are also known, which minimise the amount of spare capacity required by allowing the spare capacity on one span to contribute to the protection of other spans. Such architectures are more efficient in their use of protection bandwidth. Intelligent switching and routing operations are required to implement the protection in the event of one or more failures, and such systems are extremely computationally intensive. Mesh based architectures also require more expensive hardware, as large multiple-input/output optical switches are required. Furthermore, the response to failures is slow as a result of the required computation.
There is therefore a need for a network architecture which benefits from the shared protection capability of MESH structures, but which avoids the increased computational overhead, and preferably maintains a level of computational complexity and switching response time in line with ring-based protection systems.
According to a first aspect of the invention, there is provided a communications network comprising a plurality of nodes, which define at least two rings of nodes within the network, each ring of nodes comprising at least one working fibre and at least one protection fibre between adjacent nodes in the ring, wherein at least one adjacent pair of nodes is shared between the two rings with the working fibre and protection fibre between the pair of nodes shared between the two rings, such that working traffic on the working fibre between the pair of nodes can be protected by the ring architecture of either of the two rings of nodes, the network further comprising a network management system for routing of signals across the network, wherein the network management system dictates which rings within the network provide protection for the signal route.
The network of the invention has a ring structure, which enables protection for signal traffic on a ring to be protected by the ring architecture. Working and protection paths are shared between adjacent rings. This means it is possible for either ring to provide the protection for that path. The invention makes it possible to select the location of protection bandwidth for a signal path across the network. In this way, protection bandwidth is shared, and can also be configured to optimise the use of the total available protection bandwidth. The computational overhead required to control the provision of protection is low, because protection is based on local ring architectures within the network. However, the network is very adaptable, providing efficiency improvements, similar to MESH architectures.
Preferably, each ring defined within the network operates locally according to the BLSR protocol. The BLSR protocol, which uses K-byte SONET/SDH signalling then provides the required protection switching.
The traffic across the network may be operated on a time division multiplex basis, and a protection path may be defined in respect of individual time slots. To achieve this, K bytes for each time slot (the so-called xe2x80x9cSTSxe2x80x9d in the SONET standard) can be used for the signalling. This provides protection path switching at the granularity of the individual signal paths, rather that at the line level (as in conventional BLSR rings). Alternatively, the traffic across the network may again be operated on a time division multiplex basis, and a protection path may instead be defined in respect of a group of time slots. This enables simpler hardware to be implemented.
According to a second aspect of the invention, in a network comprising a plurality of nodes, which define at least two rings of nodes within the network, each ring of nodes comprising at least one working fibre and at least one protection fibre between adjacent nodes in the ring, wherein at least one adjacent pair of nodes is shared between the two rings with the working fibre and protection fibre between the pair of nodes shared between the two rings, such that working traffic on the working fibre between the pair of nodes can be protected by the ring architecture of either of the two rings of nodes, there is provided
a method of routing signals across the network, the method comprising:
determining a signal path between first and second nodes in the network, the path comprising path sections between nodes;
for each path section having working and protection fibres shared between rings within the network, specifying which of those rings is to provide protection for that path section.
Each path section in the network is allocated to a specific ring in the network, providing simple ring-type protection switching.
According to a third aspect of the invention, in a network comprising a plurality of nodes, which define at least two rings of nodes within the network, each ring of nodes comprising at least one working fibre and at least one protection fibre between adjacent nodes in the ring, wherein at least one adjacent pair of nodes is shared between the two rings with the working fibre and protection fibre between the pair of nodes shared between the two rings, such that working traffic on the working fibre between the pair of nodes can be protected by the ring architecture of either of the two rings of nodes, there is provided
a method of routing signals across the network, the method comprising:
determining a signal path between first and second nodes in the network, defining the signal path as a plurality of ring sections, wherein each ring section is a part of a defined ring of nodes;
for each ring section, specifying the route of a protection path for that ring section.
In this method, larger rings may be defined within the network, so that the overall level of protection bandwidth required can be reduced.